The pancreas is essential for normal digestion and maintenance of blood sugar levels. It is composed of an exocrine compartment, made up of acinar and ductal cells that secrete and transport digestive enzymes, as well as an endocrine compartment (the Islets of Langerhans), made up of alpha cells that produce glucagon, beta cells that produce insulin, delta cells that produce somatostatin, and PP cells that produce pancreatic polypeptide. We study the role of genes and signaling pathways involved in the development, function, and regeneration of specific cell types within the pancreas.
The Oc1 (aka, Hnf6) transcription factor is expressed in all pancreas cells early in embryonic development, but is "turned off" in islet cells just before birth in the mouse. We developed mice in which Oc1 is over-expressed or can be inactivated conditionally. These studies reveal that Oc1 is essential to generate the appropriate number of endocrine progenitor cells, but that it must get "turned off" in order for the insulin-producing cells to function properly. Current studies are examining how Oc1 interacts with other transcription factors in the developing pancreas to regulate endocrine differentiation and the function of mature insulin-producing beta cells. Results suggest that Oc1 is required for establishing a competency state (possibly through epigenetic modifications) in endocrine progenitors allowing for acquisition of mature function and adaptability to metabolic stressors.
A second project in the lab examines the role of CTGF, a secreted factor known to modulate growth factor signaling and affect cell proliferation and migration in other organ systems. We found that loss of CTGF results in decreased embryonic islet beta cell proliferation and defective islet formation. We are using conditional gene inactivation and over-expression strategies to determine how CTGF affects islet development and function during embryogenesis and after transplantation. In addition, we are have determined that CTGF can enhance beta cell regeneration in adults after significant beta cell destruction. We are now dissecting the signaling pathways through which CTGF affects beta cell proliferation and regeneration.
Finally, the FoxM1 transcription factor is highly expressed in proliferating cells and is essential for normal cell division. We found that FoxM1 is required downstream of all proliferative stimuli in the insulin-producing beta cells. We have identified two prostaglandin receptors as being downstream of FoxM1 to mediate its effects on beta cell proliferation and survival. Prostaglandins are important modulators of an array of physiologic functions including insulin secretion and systemic inflammation. In our lab we focus on the specific roles of EP3 and EP4 on G proteins and how they modulate inhibition or stimulation of adenylyl cyclase. Signaling via EP3 inhibits β-cell proliferation and increases β-cell death whereas activation of EP4 enhances β-cell proliferation and survival. We are currently utilizing genetic and pharmacological tools to examine the effects of EP3 and EP4 signaling in β-cell proliferation and survival in vivo and ex vivo using rodent and human islets.
Figure 5-1. Model of EP3 and EP4 signaling in mouse and human b-cells. In mouse b-cells (left), EP3 increases b-cell death and inhibits b-cell proliferation through decreased phosphorylation and activity of PLC-g1. In contrast, mouse EP4 signaling has no effect on basal or mitogen-stimulated b-cell proliferation, yet does improve b-cell survival through PKA signaling. In human b-cells (right), EP3 and EP4 have opposing effects on b-cell proliferation in the absence of any proliferative stimulus: inhibition of EP3 enhances b-cell proliferation through the activity of PLC-g1, which is further increased when EP4 is simultaneously activated. Further, EP3 increases whereas EP4 protects against b-cell death via PKA signaling. The dotted lines represent pathways with unknown intermediates.
Area of Research: Molecular and cell biology of pancreas development, function and regeneration
Research Key Words: mouse, developmental biology, molecular genetics of pancreas development, organogenesis, islet function, beta cell proliferation, beta cell survival, diabetes, transcription factor, knockout, transgenic, mutation, stem cell, tissue regeneration